The present invention pertains, inter alia, to charged-particle-beam (CPB) microlithography apparatus and methods as used for transferring a pattern, defined on a reticle, to a sensitized substrate. Such apparatus and methods have especial utility in the manufacture of integrated circuits, displays, and the like. The invention also pertains to methods and apparatus for calibrating and adjusting a CPB projection-optical system and for aligning the substrate and reticle with each other for accurate pattern transfer. The invention also pertains to methods and apparatus for reducing thermal deformation of a member, such as the reticle or a movable stage, that defines an alignment or calibration mark.
As used herein, the term xe2x80x9creticlexe2x80x9d pertains not only to reticles and masks that define an actual pattern to be transferred to a substrate, but also to aperture plates and the like as used in, for example: variable-shaped-beam projection-exposure systems, character projection systems, and xe2x80x9cdividedxe2x80x9d projection-exposure systems. In xe2x80x9cdividedxe2x80x9d projection-exposure systems, the reticle is divided or segmented into multiple xe2x80x9cexposure unitsxe2x80x9d (e.g., subfields, stripes, or other subdivisions) that are individually and sequentially exposed onto the substrate on which the images of individual exposure units are xe2x80x9cstitchedxe2x80x9d together contiguously to form the complete pattern on the substrate.
Various methods and apparatus are under current research and development for transferring, using a charged particle beam, a pattern defined by a reticle or mask onto a sensitized substrate by microlithography. Representative charged particle beams used in such systems include electron beams and ion beams. Electron-beam systems have been the subject of most such effort; hence, the following summary is in the context of electron-beam systems.
Charged-particle-beam (CPB) microlithography systems, such as electron-beam writing systems, offer tantalizing prospects of improved accuracy and resolution of pattern transfer, but exhibit disappointingly low throughput. Consequently, much contemporary research and development has focused on overcoming this disadvantage. Examples of various conventional approaches include xe2x80x9ccell-projection,xe2x80x9d xe2x80x9ccharacter projection,xe2x80x9d and xe2x80x9cblock projectionxe2x80x9d (collectively termed xe2x80x9cpartial-blockxe2x80x9d pattern transfer).
Partial-block pattern transfer is especially used whenever the pattern to be transferred to the substrate comprises a region in which a basic pattern unit is repeated many times. For example, partial-block pattern transfer is generally used for patterns having large memory circuits, such as DRAMs. In such patterns, the basic pattern unit is very small, having measurements on the substrate of, for example, (10 xcexcm)2 (i.e., 10 xcexcmxc3x9710 xcexcm). The basic pattern unit is defined on one or several exposure units on the reticle and the exposure units are repeatedly exposed many times onto the substrate to form the pattern on the substrate. Unfortunately, partial-block pattern transfer tends to be employed only for repeated portions of the pattern. Portions of the pattern that are not repeated are transferred onto the substrate using a different method, such as the variable-shaped-beam method. Therefore, partial-block pattern-transfer has a throughput that is too low, especially for large-scale production of integrated circuits.
A conventional approach that has been investigated in an effort to achieve a higher throughput than partial-block pattern-transfer methods is a projection microlithography method in which the entire reticle pattern for a complete die (or even multiple dies) is projection-exposed onto the substrate in a single xe2x80x9cshot.xe2x80x9d In such a method, the reticle defines a complete pattern, such as for a particular layer in an entire integrated circuit. The image of the reticle pattern as formed on the substrate is xe2x80x9cdemagnifiedxe2x80x9d by which is meant that the image is smaller than the pattern on the reticle by a xe2x80x9cdemagnification factorxe2x80x9d (typically 4:1 or 5:1). To form the image on the substrate, a projection lens is situated between the reticle and the substrate. Whereas this approach offers prospects of excellent throughput, it to date has exhibited excessive aberrations and the like, especially of peripheral regions of the projected pattern. In addition, it is extremely difficult to manufacture a reticle defining an entire pattern that can be exposed in one shot.
Yet another approach that is receiving much current attention is the xe2x80x9cdividedxe2x80x9d or xe2x80x9cpartitionedxe2x80x9d projection-exposure approach that utilizes a xe2x80x9cdivided,xe2x80x9d xe2x80x9cpartitioned,xe2x80x9d or xe2x80x9csegmentedxe2x80x9d reticle. On such a reticle, the overall reticle pattern is subdivided into portions termed herein xe2x80x9cexposure units.xe2x80x9d The exposure units can be of any of various types termed xe2x80x9csubfields,xe2x80x9d xe2x80x9cstripes,xe2x80x9d etc., as known in the art. Each exposure unit is individually and sequentially exposed in a separate xe2x80x9cshotxe2x80x9d or scan. The image of each exposure unit is projection-exposed (typically at a demagnification ratio such as 4:1 or 5:1) using a projection-optical system situated between the reticle and the substrate. Even though the projection-optical system typically has a large optical field, distortions, focal-point errors and other aberrations, and other faults in projected images of the exposure units are generally well controlled. Although divided projection-exposure systems provide lower throughput than systems that expose the entire reticle in one shot, divided projection-exposure systems exhibit better exposure accuracy and image resolution
In divided projection exposure, it is necessary to achieve very accurate alignment of the reticle with the substrate to ensure that the images of the exposure units are positioned at the respective locations on the reticle with extremely high accuracy. To such end, an operation termed xe2x80x9cmark detectionxe2x80x9d is performed such as during calibration of the optical system and when aligning the substrate with the reticle before exposing an exposure unit onto the substrate. During mark detection, an image of one or more xe2x80x9cupstreamxe2x80x9d marks provided on the reticle or other location on the reticle stage is projected onto a corresponding xe2x80x9cdownstreamxe2x80x9d mark provided on the substrate or other location on the substrate stage. The marks are scanned relative to each other to determine the relative positions of the marks.
Systems designed for high-resolution pattern transfer, such as the divided projection-exposure system summarized above, employ very large acceleration voltages such as between the CPB source and the reticle. To achieve the requisite high accuracy of mark detection, either mark scanning must be performed relatively slowly or a large number of scans must be performed. Consequently, the cumulative beam energy that strikes the marks and their immediate surrounding area is very high. This energy is usually dissipated as localized heating which elevates the temperature and causes thermal deformation of the vicinity of the marks. Such deformation degrades the accuracy with which mark positions can be determined, reduces calibration and alignment accuracy, and reduces the accuracy with which images of exposure units on the substrate can be stitched together. The resulting devices manufactured under such conditions exhibit a higher incidence of defects such as shorts, opens, and non-uniform resistance values.
The present invention solves certain of the problems of conventional apparatus and methods summarized above and thereby provide more accurate transfer of a reticle pattern to a substrate.
According to a first aspect of the invention, charged-particle-beam (CPB) microlithography (projection-exposure or projection-transfer) apparatus are provided. According to a representative embodiment, such an apparatus comprises an illumination optical system situated and configured to direct a charged-particle illumination beam along an optical axis from a source to a selected region on a reticle. The reticle is situated at a reticle plane orthogonal to the optical axis. The apparatus also comprises a projection-optical system situated and configured to direct a charged-particle imaging beam from the reticle to a sensitized substrate so as to transfer the pattern portion defined by the selected exposure unit to the substrate. An xe2x80x9cupstreamxe2x80x9d mark is situated on the reticle plane so as to be selectively irradiated by the illumination beam. A shield is situated between the source and the upstream mark. The shield defines an aperture that transmits a portion of the illumination beam to the upstream mark while blocking other portions of the illumination beam from reaching the reticle plane.
In the embodiment summarized above, the upstream mark can be situated on the reticle. In such an instance, the reticle can comprise multiple upstream marks distributed over the reticle. In such a configuration, the shield desirably defines multiple apertures each corresponding to a respective individual upstream mark on the reticle.
Alternatively, the upstream mark can be situated on a mark member separate from the reticle, wherein the upstream mark is situated on the mark member. In such a configuration, the shield desirably extends over the mark member. This configuration is usually used for calibration of the optics of the CPB projection-exposure apparatus.
The upstream mark can comprise multiple mark portions. In such an instance, the aperture defined by the shield can be sized, whenever the aperture is axially registered with the upstream mark, to circumscribe all the mark portions collectively. Alternatively, the shield can define multiple apertures each corresponding to a respective individual mark portion.
According to another aspect of the invention, CPB microlithography methods are provided in which a charged-particle illumination beam is used to irradiate a portion of a pattern defined by a reticle situated on a reticle plane. A projection-optical system is used to direct a corresponding charged-particle imaging beam from the irradiated portion to a sensitized substrate situated on a substrate plane. An upstream mark is defined on the reticle plane and a xe2x80x9cdownstreamxe2x80x9d mark is defined on the substrate plane. The upstream mark is selectively registrable with the downstream mark to perform beam alignment. A shield is provided upstream of the upstream mark. The shield (a) serves to block downstream passage of the illumination beam, and (b) defines an aperture having a size and profile sufficient to pass therethrough only a portion of the illumination beam sufficient to irradiate the upstream mark. When irradiating the upstream mark with the illumination beam, the illumination beam is passed through the aperture of the shield before the illumination beam reaches the upstream mark. The upstream mark can be defined on the reticle, in which instance the shield desirably extends over the reticle. Alternatively, the upstream mark can be defined on a mark member (which can be separate from the reticle), in which instance the shield desirably extends over the mark member.
In conventional CPB projection-exposure systems having utility for, e.g., performing xe2x80x9cdividedxe2x80x9d projection exposure, the illumination beam as incident on the reticle can have a transverse profile that is relatively large (e.g., (100 xcexcm)2 -(1000 xcexcm)2). A typical upstream mark is much smaller, on the order of a few xcexcm square to about a hundred xcexcm square. Whenever such upstream marks are illuminated by the charged particle beam during calibration or alignment, the beam that strikes the upstream mark is much larger in transverse area than required for illuminating the upstream mark. As summarized above, the resulting large amount of energy being dissipated in an area surrounding the upstream mark can cause thermal deformation of the upstream marks. Whereas it might be possible to reduce the transverse area of the beam, such a method is impractical because it requires a very complex irradiation optical system. Apparatus and methods according to the invention, as summarized above, reduce the transverse area of the illumination beam actually irradiating an upstream mark, thereby largely eliminating thermal deformation of the mark(s).
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.